Poly-phase DC motors, and more particularly three-phase DC motors of the brush-less, sensor-less type, are widely used in computer system disk drives, such as floppy disk, hard disk, or CD ROM drives, as well as in other applications. Such motors can be driven in two different control modes: pulse width modulation or linear.
In many systems, there is need to switch between pulse-width modulation control mode to linear control mode. In some applications, this switching is done once per revolution of the motor. In other applications, the control mode is switched several times per revolution. In still other applications, the motor is primarily driven in linear mode and occasionally in pulse width modulation mode. Frequently, this switching occurs at a selected time in the revolution of the motor. Often, the selected time is when the voltage induced in the coil that is floating, i.e., the back EMF voltage, exhibits a zero crossing point.
One problem encountered with switching from PWM control mode to linear control mode is that linear control circuitry requires a finite time to stabilize after switching to achieve reliable operation. This settling period is in part because the bias circuit of the linear control circuitry tends to partially discharge while the controller is operating in pulse width modulation mode, and requires time to be recharged to an appropriate voltage.